I feel like I'm asking a FAQ but I couldn't find it. If I am feel free to kick me in the right direction.

How many devices really work with the USB ports? I have a Memorex wireless mouse/KB combo. Will this just work or does there have to be a driver for each device? What do the serial and parallel ports do for you besides allow flash recovery? I'm a programmer but not a hardware guy, so would it be useful for software debugging and stuff? What about USB hard drives? Do they work? What are the size and file system limitations? How bad will the cradle hurt battery life?

TIA. My GP2X has kind of been collecting dust as of late and this might be an excuse to whip it out again.

Your keyboard/mouse are 99% likely to work (On software that supports them) without extra power. Yes, USB hard drives should work, they need to be formatted FAT32 and I don't know of any size limitations. However if you want to connect a hard drive, you can't power it with just the GP2X batteries, you need to either supply the cradle power from an adaptor, or the hard drive.

Emulators that work with USB Joysticks:

PicoDrive
MAME
GneoGeo
FCEUltra
CPS2
AlexKidd

You forgot GP2Xpectrum!

I've never tried it

You guys are quick with the replies.

So what about one of those PS2 to USB adaptors? I have this guy: http://cgi.ebay.com/PS-PS2-USB-Dual-Player...1QQcmdZViewItem

Being able to use PS2 controllers on a GP2X would be worth it right there. How easy would it be to incorporate cradle features into my own code should I get that far?

I've heard those work.
Also, I have a USB joystick from ledshoppe.com, it cost only 10$ and is identical to the PS/PS2 joystick, just USB.

My 10-button logitech precission gamepad, while in standart gp2x menu reacts to right on d-pad as pressed stick.

I've had trouble with a particular no-name PSX to USB dual port adapter; firstly it needs a full 5V power supply to work, and the Dpad directions are not registered in any of the USB software. However my ancient Gravis Gamepad Pro works 100% flawlessly...too bad its such a crappy pad.

It makes a great ass wedge.